My personal pain point with JS as a functional language is its lack of native persistent data structures and the fact that its mutable-by-default nature has spa

My personal pain point with JS as a functional language is its lack of native persistent data structures and the fact that its mutable-by-default nature has spawned an ecosystem of libraries and frameworks that overwhelmingly exhibit reckless disregard for managing mutable state in a sane manner. The lack of native persistent data structures means functional programs need to depend on libraries like ImmutableJS and Mori to fill in the gap, but in doing so they have to trade away language features like spread and destructuring that improve readability and minimize boilerplate, and take a hit to ease of debugging. I almost always pick up ImmutableJS or Mori for my JS projects these days despite these issues, so it's certainly not the end of the world, but whenever I work with them it's painfully clear that persistent data structures, and by extension functional programming, is a second class citizen in the context of the language. The ecosystem issue is a much harder one to work around however. You can take every precaution to carefully craft your code to minimize side effects and manage state transitions, but there's only so much you can do if your components/visualization/networking library (mis)manages its own internal state. The situation has improved dramatically in the past few years with the advent of React/Redux, and we've experienced a functional programming renaissance with tons of great new libraries popping up that are amenable to functional paradigms, but we're only just starting to make a dent in the massive JS ecosystem, which is still overwhelmingly inadequate when it comes to dealing with mutable state and managing side effects. All of these pain points become painfully obvious once you've worked with an ecosystem that has spawned from a functional-first language that is immutable-by-default and encourages being mindful of state and side effects (in my case, that language was Clojure and ClojureScript, but that's certainly not the only one).